Kennedy, 81, spent more than a decade as the court's most frequent tie-breaker, taking on that role after Justice Sandra Day O'Connor retired in 2006. In the years since, Kennedy fully embraced his role on an ideologically split court. Not bad for a man who was President Ronald Reagan's third choice when a high-court seat came open in 1987.
Kennedy's legal reasoning sometimes made his critics, mainly on the right, sputter. "Legalistic argle-bargle" was conservative colleague Antonin Scalia's description of the reasoning in Kennedy's opinion striking down part of the federal anti-gay marriage law in 2013.
The California native wrote the Supreme Court's four most significant gay rights cases and was also a key player in the court's decision to reaffirm a woman's right to an abortion. He also wrote opinions that reined in the use of the death penalty and gave suspected terrorists detained at Guantanamo Bay, Cuba, the right to plead for their freedom in civilian courts.
At the same time, Kennedy wrote the Citizens United decision in 2010 that upended campaign finance restrictions and was a fifth vote in landmark cases granting gun rights and striking down part of the Voting Rights Act.
In his final term, he did not once join the four liberals in the majority of a closely divided case. He was part of conservative majorities to uphold Trump's travel ban, deal labor unions a major financial setback, approve Ohio's aggressive purge of its voter rolls and prohibit millions of workers from banding together to complain about pay.
Scholars struggled to find a thread that ran through all his votes, but no one doubted his influence. One of the few big cases in which Kennedy was on the losing side was the court's consideration of President Barack Obama's health care overhaul in 2012. Playing Kennedy's usual role, Chief Justice john Roberts joined with the court's liberals to uphold the law.
Far from an ideologue, Kennedy seemed to wrestle with every decision the court was asked to make. "We, of course, are bound by the facts, the law, the rules of logic, legal reasoning and precedents," he once said. "But we are also bound by our own sense of morality and decency. ... We must never lose sight of the fact that the law has a moral foundation, and we must never fail to ask ourselves not only what the law is, but what the law should be."
Kennedy also worried about judges taking too active a governing role. "No society should leave it to a court to make most of its decisions," he once told an acquaintance. "No court can live up to that."
Kennedy's sometimes-public brooding about his role was derided by some legal scholars. His critics said that his decisions in so many of the court's high-profile cases belied his rhetoric about limiting the courts' role. That was the thrust of Scalia's dissent in the 2013 gay marriage case in which Kennedy wrote a majority opinion that was joined by his four liberal colleagues.
That opinion in U.S. v. Windsor held that legally married gay couples must receive the same federal benefits as all other married couples and decried the second-class status of same-sex couples, even in states that granted them the right to marry. It was followed two years later by Obergefell v. Hodges, the decision that said same-sex couples have a right to marry anywhere in the United States.
"No union is more profound than marriage, for it embodies the highest ideals of love, fidelity, devotion, sacrifice, and family. In forming a marital union, two people become something greater than once they were," Kennedy wrote in that decision before adding that gay couples were asking for "equal dignity in the eyes of the law." ''The Constitution grants them that right," Kennedy wrote.
The Obergefell case was the fourth in a series of opinions in which Kennedy delivered forceful, and seemingly heartfelt, majority opinions in favor of gay rights. In 1996, he wrote for the court in overturning an anti-gay Colorado constitutional amendment. In 2003, Kennedy again led the way in striking down state bans on gay sex in the case of Lawrence v. Texas, a ruling that Scalia and others said would pave the way for same-sex marriage.
"Kennedy has now firmly secured his place in history as 'the first gay justice,'" Cornell University law professor Michael Dorf, a former Kennedy law clerk at the Supreme Court, blogged the day of the Windsor decision.
That was not a moniker many would have predicted for Kennedy when he joined an ideologically divided court in 1988. His nomination followed the failed and bruising effort to put Robert Bork on the court and the withdrawal of Douglas Ginsburg after reports surfaced he smoked marijuana while a law professor. Kennedy was viewed warily by some in Reagan's circle as insufficiently conservative, but he was well-known and liked by two fellow Californians, the president himself and Reagan confidant Edwin Meese, then the U.S. attorney general.
At first, Kennedy was a steady ally for Chief Justice William H. Rehnquist and other conservatives, delighting conservative scholars and confirming the fears of women's and liberal groups. Yet it wasn't long before Kennedy gradually showed greater independence, increasingly disagreeing with the court's conservatives on social issues such as abortion, freedom of religion and freedom of expression.
When a news article suggested that Kennedy was undergoing the same transformation as conservative-turned-liberal Justice Harry A. Blackmun, Blackmun wrote to Kennedy: "Don't worry. It's not fatal." Two rulings in June 1992 both confirmed his independence and gave rise to a perception among some critics that Kennedy gave in to pressure from a liberal cultural elite.
In the first case, Kennedy initially joined with his conservative colleagues in believing that there was no constitutional problem with a rabbi giving a prayer at a public school graduation ceremony. In the end, Kennedy switched sides and led the court in ruling that public school officials violate the constitutionally required separation of church and state when they invite clergy members to say prayers — invocations and benedictions — at graduation ceremonies.
Just a few days later, Kennedy was part of a majority to reaffirm the court's Roe v. Wade decision declaring a woman's constitutional right to abortion. The opinion in Planned Parenthood v. Casey was an unusual collaboration of three justices — O'Connor, Kennedy and David Souter. In that case, too, Kennedy first sided with and provided a majority to justices who would have effectively overruled Roe.
But Kennedy, a Roman Catholic with antipathy to abortion, told an interviewer that he couldn't let his personal distaste for abortion determine his vote. Fifteen years later, Kennedy wrote for a conservative majority to uphold a federal law banning a certain abortion procedure known as partial-birth abortion, drawing a fierce dissent from Justice Ruth Bader Ginsburg. Kennedy's opinion gave hope to abortion opponents and fueled a drive to pass new restrictions on abortion in many states. But the court did not, after 1992, come close to questioning the abortion right first announced in 1973. Indeed, in 2016, it struck down some restrictions on abortion clinics in Texas, with Kennedy and the four liberals forming the majority.
Before taking Lewis F. Powell's seat on the Supreme Court, Kennedy was a federal appellate judge based in his hometown of Sacramento, California, for 12 years and taught constitutional law. Earlier, he practiced law and worked as a lobbyist in the California Legislature. He and his wife, Mary, have three children.
Bork, Reagan's first choice, was rejected by the Senate after a bitter political struggle in which the conservative Bork was portrayed as a rigid extremist. But Kennedy was confirmed without an opposing vote.
It is perhaps hard to overstate how different the court will be without Kennedy. Court commentator Tom Goldstein once said, "It's Justice Kennedy's world. We all just live in it."